With the development of city, the amount of vehicles has increased significantly. The emissions from vehicles have become an important cause of air pollution such as haze. Due to high boiling point and low H/C mass ratio, the smoke and soot formation (including particulate matter, nitrogen oxides, CO, etc.) of diesel is more serious than that of gasoline during combustion. In order to reduce the air pollution caused by vehicles, it is important to upgrade the combustion performance of diesel.
Oxygenated compounds with few or no C—C bonds can greatly improve the efficiency of diesel combustion and reduce smoke and soot formation when added to conventional diesel fuel. A large number of oxygenated chemicals like ethers, acetals, alcohols and lipids have been widely studied to be used as diesel fuel additives. However, no oxygenates have been widely applied because they do not meet the ideal characteristics of an oxygenated compound to blend with diesel which includes an adequate cetane number, a high boiling point to satisfy the flash point specifications, a low condensation point to guarantee good cold flow properties, being miscible with various types of diesel fuels and a suitable density. Other properties like toxicity, biodegradability, environmental friendliness, sustainability, raw material adequacy and so on should also be strictly taken into consideration.
Besides air pollution, oil resources exhaustion is another issue that needed to be addressed as a matter of urgency. Finding alternative energy sources to replace the depleting oil resources is imperative. With the advantage of abundant source, environmental friendliness and being easy to store and transport, methanol is considered to be the best alternative energy sources. Rich in coal but poor in natural gas, China has pioneered the development of coal-based methanol economy and Cl chemicals like methanol and formaldehyde is facing serious oversupply in recent years while diesel often encounters short supply due to crude oil shortage and seasonally soared consumption. Therefore synthesis of oxygenated compounds as diesel fuel additives from methanol is of great interest. This can fully utilize the large surplus Cl chemicals and alleviate the diesel supply crisis, and can bring enormous economic and environmental benefits.
Methanol, together with dimethyl ether and dimethoxymethane produced from Cl chemicals have all been considered to be used as diesel fuel additives. However, Methanol has the disadvantages of low solubility in diesel and low cetane number. Dimethyl ether has a high cetane number, but its addition to diesel increases the vapor pressure and lowers the viscosity. Especially, dimethoxymethane (DMM) with the ability to significantly reduce smoke and engine exhaust emissions, has drawn much attention. However, DMM has a low cetane number and is prone to cause vapor lock. Therefore, methanol, DME and DMM are difficult for wide use as diesel fuel additives.
Moulton et al. disclosed in U.S. Pat. No. 5,746,785A that the fuel containing the mixed polyoxymethylene dimethyl ethers (PODEn) blend component is safer to handle and use than fuel containing the same amount of dimethoxymethane. It was also found that in comparison with the diesel fuel containing dimethoxymethane alone as a blended component, the diesel fuel containing mixed PODEn is less volatile, has a higher flash point, has a higher viscosity closer to that of conventional diesel fuels and has higher fuel lubricity. All these properties make PODEn ideal diesel fuel additives.
PODEn (CH3O(CH2O)nCH3) refers to a homologous series of oxygenated compounds. Among the PODEn compounds, PODE2 does not satisfy the security criterion due to its low flash point, and PODEn>5 will precipitate at low temperatures due to high melting point. The PODE3˜5 compounds are most ideal diesel additives because their physical properties are consistent with those of diesel fuels and the oxygen content (˜50%) and cetane number (70 to 100) are high. Directly blending the diesel fuel with 20% (v/v) PODE3˜5 can improve the combustion efficiency of fuel while at the same time alleviate the diesel short supply, bringing important environmental and economic benefits.
Some processes have been proposed for preparation of polyoxymethylene dimethyl ethers PODEn.
U.S. patent references U.S. Pat. No. 5,959,156A, U.S. Pat. No. 6,160,174A, U.S. Pat. No. 6,160,186A, U.S. Pat. No. 6,392,102B1 by British Petroleum (BP) describe a process in which methanol or dimethyl ether is converted to formaldehyde via oxidative dehydrogenation, and then formaldehyde reacts with methanol or dimethyl ether forming dimethoxymethane and polyoxymethylene dimethyl ethers. The process is very complex, comprising unit operations including oxidative dehydrogenation, adsorption cooling, catalytic distillation, neutralization and separation. The selectivity to PODE.sub.n>1 in polyoxymethylene dimethyl ethers is less than 10%.
U.S. patent references U.S. Pat. No. 7,700,809B2, US20070260094A1, and U.S. Pat. No. 7,671,240B2 by BASF describe the preparation of polyoxymethylene dimethyl ethers from dimethoxymethane and trioxane in the presence of acidic catalyst. The selectivity to PODE.sub.3.about.5 in polyoxymethylene dimethyl ethers is about 20 wt %, owing to the low water content in system (<1%). However, the cost of highly purified trioxane and dimethoxymethane is too high. Besides, a considerable amount of by-products PODE.sub.n>5 are produced, thus complicating the separation process.
U.S. patent references US20100056830 A1 and U.S. Pat. No. 7,560,599B2 by Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, describe the preparation of polyoxymethylene dimethyl ethers from methanol and trioxane in the presence of acidic ionic liquid. The conversion of trioxane could reach 90%. However, ionic liquid is unfavorable due to high cost, difficult separation and recycling, thus complicating the process.
Chinese patent application publication of CN103360224A by Runcheng Carbon Material Technology Co. Ltd in Dongying, China, describes a process for producing polyoxymethylene dimethyl ethers. In this process, the reactants are circulated in the reactor and membrane separation device, aiming for 100% conversion of formaldehyde. However, the energy cost of this process is huge. The attempt to reach 100% conversion of formaldehyde is hard to realize due to the reversibility of the reactions forming polyoxymethylene dimethyl ethers.
Chinese patent application publication of CN102701923A by Beijing Coreteam Engineering & Technology Co. Ltd., describes a process for preparation of polyoxymethylene dimethyl ethers from methanol and trioxane using ionic liquid as catalyst in a cannula reactor. The process is very complex, comprising the reaction unit, vacuum flashing unit, extraction unit, alkali washing unit, and rectification unit. In this process, the yield of target product is low while the ionic liquid catalyst is hard to recycle. In the alkali washing unit, alkali could react with the unconverted formaldehyde, thus increasing feedstock cost.